Processes for the separation of air to produce nitrogen are known, as disclosed for example, in Ruhemann et al., U.S. Pat. No. 3,203,193 and Keith, Jr., U.S. Pat. No. 3,217,502. These processes provide for the operation of the single distillation column at a slightly higher pressure than the product delivery pressure. Air is introduced at the column sump. The oxygen enriched liquid air withdrawn from the distillation column flashes in indirect heat exchange with a portion of the overhead nitrogen product. In addition, plant refrigeration is provided by further expanding the vaporized oxygen enriched air, also termed "waste nitrogen." Such methods are able to recover up to about 35 to 40 mole percent of the feed air as nitrogen product.
Patel et al., U.S. Pat. No. 4,400,188, discloses the use of a heat pump to separate nitrogen. The process, however, is only cost effective for the production of very large quantities of nitrogen, e.g. 15 to 200 million standard cubic feet/day (SCFD) (that is, 625,000 to 8,000,000 SCFH). The process uses overhead vapor recompression to enhance separation which requires complex and costly equipment making it uneconomical for recoveries in the range of less than about 15 million SCFD (625,000 SCFH).
Conventional single distillation column systems, which expand waste nitrogen in a turboexpander for refrigeration generally filter and compress the feed air to above the nitrogen delivery pressure. The air is purified of its carbon dioxide and moisture contents by adsorptive means, such as molecular sieves, and then cooled to near its dew point temperature. Alternatively, carbon dioxide and condensed moisture are removed in a reversing heat exchanger, in which the air and waste stream passages can be alternated, which allows the deposited impurities to evaporate into the waste stream which is ejected to the atmosphere.
The cooled air stream is fed to a distillation column where it is separated into an oxygen-rich liquid at the base of the column and a substantially pure nitrogen gas stream at the top. A portion of the pure nitrogen gas is warmed to ambient temperature and delivered as product. The balance is sent to a condenser to provide column reflux. Vaporized oxygen-rich liquid (typically termed "waste nitrogen") from the condenser is warmed in a heat exchanger and then expanded in a turboexpander to provide refrigeration for the system. Such systems characteristically recover only about 35-45 mole percent of the feed air as nitrogen product. It is therefore a significant advance in the art if the mole percent recovery of nitrogen from the feed air is significantly increased.